There has been significant interest in using membranes separation for both industrial scale processes and the preparation of analytical samples. In part, this is due to the fact that membranes facilitate extraction and separation without allowing the mixing of two phases. Membrane distillation (MD) is a low temperature (60-90° C.) distillation process that has been used in diverse applications ranging from water desalination to concentration of fruit juice and sucrose. Conventional distillation relies strictly on thermal vaporization, whereas in MD the water vapors are selectively transmitted through a microporous membrane. The heated solution is passed through the lumen of a hydrophobic hollow fiber which prevents passage of the liquid phase. However, the hot solution has a high vapor pressure and MD relies on the net flux of water vapor from the warm side of the membrane to the cool side. The driving force for the vapor transport is determined by the vapor pressure difference across the membrane, which depends upon the temperature differences. Analytical applications of membrane extraction have relied on the selective permeation of the analyte, and this is a challenge because selective membranes for diverse analytes are not always available.
Carbon nanotubes (CNTs) have been used in a variety of applications related to separations and concentration of trace impurities. Various studies have shown that both liquid and gas permeability through CNTs far exceed what is expected from traditional diffusion techniques. Use of nanoparticles with polymeric membrane materials have also been shown to have enhanced separation in assorted liquid and gaseous systems.
To date, however, CNTs have not been employed in connection with membrane distillation techniques.